Radiation source containers, such as contained irradiators, shipping casks and the like that contain radioactive material, such as cesium-137 or cobalt-60, are subject to a difficult design problem. Most of these devices are constructed of steel and/or lead, and although components can be fabricated with some degree of precision, it is still necessary to have components that move relative to each other, for example a movable or removable closure system for the radiation source container. In such instances, it is desirable or necessary to provide fairly large tolerances to accommodate considerable expansion and contraction, and to avoid a "tight fit" to facilitate assembly, in many cases by robotic equipment. This in turn results in cracks or gaps between adjacent faces on adjoining components. Radiation from the sources will "stream" through these cracks escaping from the unit, unless means are employed to prevent such escape.
The most obvious manner of attempting to reduce radiation escape is to keep the gap between the components as small as possible. This approach is limited by the possibility that the components may collide and cause binding. This binding or jamming can in turn require that the unit be repaired, usually remotely in a hot cell or water pool, which is both inconvenient and quite expensive. Therefore, the designer will want to keep the component interface distance as large as possible and yet meet the radiation integrity requirements.
The most commonly used manner of preventing radiation streaming is by the use of "steps," as illustrated in prior art FIG. 3. Gamma photons travel in straight lines and, unlike visible light photons, there is very little reflection off surfaces on impact. A typical reflection albedo is in the range of 1%. The use of stepped gaps or passages, whether angular as in FIG. 3 or arcuate as in prior art FIG. 4, is very effective in reducing streaming. The steps are set perpendicular to the direction of photon travel, and on impact, most of the photons are absorbed by the material of the encountered surfaces where they are converted to low grade heat. The rest of the photons scatter. A small percentage are "reflected" and stream on through the gap until they impact the second turn in the step and the process is repeated. This traps even more photons. The curved joinder of FIG. 4 functions in basically the same manner.
While the stepped shield is very effective, it is not usually in itself sufficient. For example, it is sometimes necessary to reduce radiation levels from the inside of an irradiation chamber to the outside by a factor of more than a billion. Multiple steps are helpful, but present additional design problems, and can complicate assembly. Further, as schematically suggested in FIG. 5 at A, the laterally angling random radiation flux is substantially unimpeded between the opposed planar faces.
Other approaches have been proposed, such as filling the gaps with mercury, thus forming a continuous high density fluid shield between components. However, mercury vapor is toxic, and an inadvertent leak of this fluid would breach shield integrity. This could occur by simply turning a portable radiation source container, such as a cask, upside down.